1. Field of the Invention
Embodiments disclosed herein generally relate to a plasma enhanced chemical vapor deposition (PECVD) apparatus.
2. Description of the Related Art
PECVD is a process whereby a gas is introduced into an apparatus, the gas is ignited into a plasma and a material layer is deposited onto a substrate. There are many applications for utilizing a PECVD process such as to deposit layers for a flat panel display (FPD), to deposit layers for a solar panel and to deposit layers for an organic light emitting display (OLED) to name a few.
For FPD applications in particular, the PECVD chamber size has increased over the years to accommodate the consumer demand for larger FPDs. As the chamber size has increased, so has the power applied to generate the plasma. These larger chambers encounter numerous problems that smaller chambers, typically used to process semiconductor substrates, may experience such as RF grounding and plasma distribution to name only a few. However, the magnitude of the problems for large chambers is significantly greater than for smaller chambers, and the solutions to the problems are not necessarily the same for small chambers and large chambers.
One of the problems faced by large area processing chambers is the effect of slit valve openings. The slit valve opening through one of the chamber walls affects the RF return path and may lead to plasma asymmetry within the processing chamber. As the slit valve opening becomes larger to accommodate even larger substrates, the asymmetry can increase. The asymmetry affects the film thickness and the film properties.
FIG. 1 is a cross sectional view of a prior art large area PECVD apparatus. The apparatus includes a chamber 100 in which one or more films may be deposited onto a substrate 120. The chamber 100 generally includes walls 102, a bottom 104, a showerhead 106, and substrate support 118 which define a process volume. The process volume is accessed through a slit valve opening 108 such that the substrate 120 may be transferred in and out of the chamber 100. The substrate support 118 may be coupled to an actuator 116 to raise and lower the substrate support 118. Lift pins 122 are moveably disposed through the substrate support 118 to move a substrate to and from the substrate receiving surface. The substrate support 118 may also include heating and/or cooling elements 124 to maintain the substrate support 118 at a desired temperature. The substrate support 118 may also include RF return straps 126 to provide an RF return path at the periphery of the substrate support 118.
The showerhead 106 is coupled to a backing plate 112 by a fastening mechanism 150. The showerhead 106 and the backing plate 112, because they are coupled to the RF power source 128 are considered to be electrodes. The showerhead 106 may be coupled to the backing plate 112 by one or more fastening mechanisms 150 to help prevent sag and/or control the straightness/curvature of the showerhead 106. In one embodiment, twelve fastening mechanisms 150 may be used to couple the showerhead 106 to the backing plate 112. The fastening mechanisms 150 may include a nut and bolt assembly.
A gas source 132 is coupled to the backing plate 112 to provide gas through gas passages in the showerhead 106 to the substrate 120. A vacuum pump 110 is coupled to the chamber 100 to control the process volume 106 at a desired pressure. A RF power source 128 is coupled to the backing plate 112 and/or to the showerhead 106 to provide a RF current to the showerhead 106. The RF current creates an electric field between the showerhead 106 and the substrate support 118 so that a plasma may be generated from the gases between the showerhead 106 and the substrate support 118. Various frequencies may be used, such as a frequency between about 0.3 MHz and about 200 MHz. In one embodiment, the RF current is provided at a frequency of 13.56 MHz. The RF current is supplied to the chamber 100 through a match network 160. The match network 160 is coupled to the gas feed through 170 by an electrically conductive strap 162 and a connection plate 164. The RF current travels from the match network 160 along a path shown by arrows 166 and returns along the path shown by arrows 168.
A remote plasma source 130, such as an inductively coupled remote plasma source 130, may also be coupled between the gas source 132 and the backing plate 112. Between processing substrates, a cleaning gas may be provided to the remote plasma source 130 so that a remote plasma is generated. The radicals from the remote plasma may be provided to chamber 100 to clean chamber 100 components. The processing gas or remotely generated radicals travel through a gas feed through 170 before entering the area between the backing plate 112 and the showerhead 106.
The backing plate 112 may be supported by a support assembly 138. One or more anchor bolts 140 may extend down from the support assembly 138 to a support ring 144. The support ring 144 may be coupled with the backing plate 112 by one or more fastening mechanisms 142. In one embodiment, the fastening mechanisms 142 may comprise a nut and bolt assembly. The support ring 144 may be coupled with the backing plate 112 substantially in the center of the backing plate 112. The center of the backing plate 112 is the area of the backing plate 112 with the least amount of support in absence of the support ring 144. Therefore, supporting the center area of the backing plate 112 may reduce and/or prevent sagging of the backing plate 112.
The showerhead 106 may be coupled to the backing plate 112 by a bracket 134. The bracket 134 may have a ledge 136 upon which the showerhead 106 may rest. The backing plate 112 may rest on a ledge 114 coupled with the chamber walls 102 to seal the chamber 100. A chamber lid 152 may be coupled with the chamber walls 102 and spaced from the backing plate 112 by area 154. The chamber lid 152 may have an opening therethrough to permit the one or more fastening mechanisms 142 to couple with the backing plate 112 and the gas feed conduit 156 to supply processing gas to the chamber 100.
An RF return plate 146 may be coupled with the ring 144 and the chamber lid 152. The RF return plate 146 may be coupled with the chamber lid 152 by a fastening mechanism 148. The RF return plate 146 may be coupled between the fastening mechanism 142 and the ring 144. The RF return plate 146 provides a path for the RF current to flow back down to the chamber lid 152 and then to the RF power source 128.
FIG. 2 is a schematic isometric view of a prior art center feed RF coupling. As shown in FIG. 2, it is quite crowded in the center area. FIG. 2 shows the matching network 160 coupled to an end block 210. The electrically conductive strap 162 is coupled to the match network 160 and the connection plate 164 is coupled to the end block 210. Gas is fed to the chamber from a gas source through the gas feed through 170 into the end block 210 and into the chamber. The end block 210 is surrounded by the support ring 144 that couples the backing plate to the support structure.
As discussed above, the processing chamber 100 may have an asymmetry due to the slit valve that causes an asymmetry in film thickness and properties. By moving RF feed location from the center, the asymmetry may be controlled. The location may be moved closer to the slit valve opening and thus, counteract the slit valve opening.
Another manner of cancelling the slit valve opening effect on the plasma is to utilize multiple feeding points for the RF power to couple to the electrode. However, multiple feeding points may lead to reliability problems even with a small variation in the configuration. For example, the impedance of both feeding extensions in a dual feeding system can be changed easily by stray capacitance to the grounding wall. The stray capacitance will cause current change and affect the process results. Another possibility is that capacitors installed in the middle of the RF feeding strap could be different within manufacture's error range. In other words, the process is not consistent even with a small variation from chamber to chamber. Thus, a single feed has typically been used because it provides greater consistency as opposed to a multiple feed that will have an unbalanced current.
The sensitivity of the film profile by the feeding location is a function of the chamber size and the process gases among other things. Therefore, the feeding point is usually different based on the chamber configuration and process. In other words, some cases utilize a feeding point that is very close to the center of the electrode while others utilize a feeding point quite far from the center. Thus, even though a single feed location closer to the slit valve opening may be used to counteract the effects of the slit valve opening, there are situations where there is a competing interest in maintaining the single feed location as close to the center of the electrode as possible. A single feed location that is closer to the slit valve opening and also at the substantial center of the electrode is not possible. A multiple feed option is attractive; however, the multiple feed could be inconsistent as discussed above.
Therefore, there is a need in the art for a PECVD apparatus that has multiple locations where the RF power source couples to the electrode and decreases plasma asymmetry.